FRIDAY, Feb. 1 (HealthDay News) -- A 45-minute midday nap can help boost your memory and remember facts, but only if you learned them well in the first place, a new study suggests.

This type of memory is called "declarative memory" and applies to standard textbook learning and knowledge, in contrast to "procedural memory," which applies to skills. Sleep appears to help "set" these declarative memories and make them easier to recall, the researchers said.

"Sleep appears to have an impact on what is learned well, but not so much when one is not motivated to learn," said lead researcher Matthew A. Tucker, a postdoctoral fellow at Harvard Medical School's Center for Sleep and Cognition.

For the study, 33 people were trained with certain declarative memory tasks. After the training, 16 took a non-REM nap, while 17 stayed awake and watched a movie. Later the same day, all the participants were tested. The tests included memorizing words, memorizing a maze and memorizing a complex line drawing.

Tucker's team found that over three very different declarative memory tasks, taking a nap improved performance compared with staying awake. However, napping only worked for people who had really learned the task well in the first place.

"The nap group performed better overall than the awake group, but the difference wasn't significant," Tucker said. "However, when we looked at individual performance during training, we found those who did better during training benefited from napping," he said.

In addition, people appeared to perform well on one task only, but not all three, Tucker said. "There is likely a basic level of learning that has to be attained before sleep can have an impact on performance," he said.

The findings were published in the Feb. 1 issue of the journal Sleep.

Tucker thinks that taking a nap may actually improve one's memory of facts if one is motivated to learn. "There is a lot of data starting to come in that there are benefits from naps on memory," he said.

Sara Mednick, an assistant professor at the University of California, San Diego's Laboratory of Sleep and Behavioral Neuroscience, said the new study is further proof of the role of sleep on memory and learning.

"This paper is further evidence of how sleep, specifically naps, can be a tool for memory consolidation," she said. "Interestingly, the data shows that not all subjects utilize sleep for consolidation to a similar extent."

More information

For more on the importance of sleep, visit the National Sleep Foundation.

SOURCES: Matthew A. Tucker, Ph.D., postdoctoral fellow, Center for Sleep and Cognition, and the department of psychiatry, Harvard Medical School, Boston; Sara Mednick, Ph.D., assistant professor, Laboratory of Sleep and Behavioral Neuroscience, University of California, San Diego; Feb. 1, 2008, Sleep

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